[iguy]

In the West the previous system was not arranged marriages, it was just monogamy.

If the "top 20%" (in the example above) married the opposite "top 20%" and this permanently removed them from the pool of available young people, then the matching could proceed on the rest. But if instead they form temporary bonds, then the system is more complicated, and you can have a state where (perhaps) the top 50% of one side take turns with the top 20% of the other. By "take turns" (apologies if it's a strange phrase) I mean both medium term dating and marriage with divorce, both common patterns now (e.g. some people are coupled most years of their 20s, some just a few).